High acoustic transmission loss panel



May 3, 1966 B. G. WATTERS 3,249,178

HIGH ACOUSTIC TRANSMISSION LOSS PANEL Original Filed Nov. 5, 1959 LONGITUDINALLY SHEARABLE, STIFF SHEETS TRANSVERSELY INCOMPRESSIBLE,

ELASTIC LAYERS INVENTOR BILL 'G. wATTERS ATTORNEYS United States Patent 3,249,178 HIGH ACOUSTIC TRANSMISSION LOSS PANEL Bill G. Watters, Nahant, Mass., as'signor to Bolt, Beranek and Newman, Inc., Cambridge, Mass., a corporation of Massachusetts Original application Nov. 5, 1959, Ser. No. 851,099, now Patent No. 3,087,574, dated Apr. 30, 1963. Divided and this application Sept. 24, 1962, Ser. No. 225,922

3 Claims. (Cl. 181-33) The present invention relates to walls, partitions, and other' surfaces, hereinafter generically referred to as panels, for providing a high acoustic transmission loss over a wide band of acoustic frequencies, the present application being a division of copending application Serial No. 851,099, filed November 5, 1959, for High Acoustic Transmission Loss Panel and the Like, now Patent #3,087,574, dated April 30, 1963.

As is explained in the said application, and in my article entitled New Wall Design for High Transmission Loss or High Damping, appearing in the Journal of the Acoustical Society of America, volume 31, No. 6, pages 739- 748, sandwich-type panels may be constructed that provide a high acoustic transmission loss.

An object of the present invention is to provide a novel type of sandwich panel operating in accordance with the acoustic principles above mentioned and that comprises a plurality in excess of two layers of stiff sheet material, with each layer separated from but connected to the adjacent layer by an intermediate layer of elastic material that is preferably shearable, but substantially incompressible.

A further object is to provide a novel light-transmitting panel of more general utility, also.

Other and further objects will be explained hereinafter and will be more particularly defined in the appended claims.

The invention will now be described in connection with the accompanying drawing, the single figure of which is a fragmentary longitudinal section of a panel constructed in accordance with a preferred embodiment of the invention.

The criteria set forth in the above-mentioned co-pending application and article for producing the phenomena required in order to achieve high transmission loss, are several fold. First, the velocity of the effective shear wave propagating in the interior medium of the panel between the outer surfaces or facings, must not exceed approxi mately 0.7 of the velocity C of sound in the air or other medium surrounding the panel; preferably not greater than two-thirds such velocity. This requires a sufficiently low ratio of shear modulus-to-density of the interior medium of the panel and the like, where density is defined as the density of the interior medium, as loaded with the panel structure. Secondly, the effective longitudinal stiffness of the interior medium, defined as the longitudinal stiffness of the shearable or effectively shearable part of the interior medium, must be less than the longitudinal stifiness (Youngs modulus times thickness) of the inner and outer facing surfaces in order to set up the shear-wave boundary condition essential to the attainment of the above-described phenomenon. Thirdly, if it is to be assured that the acoustic transmission loss be not reduced within the desired band of acoustic frequencies by the phenomenon of mechanical double-wall resonances, set up transversely between the inner and outer panel surface facings, the thickness of the panel and the parameters of the interior medium between the outer surfaces must be such as to provide a transverse mechanical resonant frequency outside the band of acoustic frequencies that are to be prevented from transmission through the panel and the like. i

Patented May 3, 1966 Referring to the drawing, a panel satisfying the abovementioned criteria is illustrated as constructed of relatively inexpensive materials and is of simple structural design. The panel comprises a plurality of layers 3, 5, 3, of stiff sheet material, with the layers 3 and 3' forming the outer surfaces of the panel. Each of the layers is separated from but connected to the adjacent layer by an intermediate layer 4 of elastic material, forming a sandwich construction.

As stated in the said co-pending application, the layers 3, 5, 3', may be of stiff metal, glass, mica or the like and the elastic layers 4may be of rubber, synthetic elastomers, appropriately plasticized plastic materials, suchas polyvinyl chloride and similar soft but substantially imcompressible viscoelastic materials, connected or adhered to the stiff layers with bonding adhesive. In the case of glass layers and certain viscoelastic layers, including polyvinyl chloride, of course, the panel may be inherently light-transmitting, which is desirable for some applications.

The reason for selecting an elastic material for the intermediate layers 4 that is shearable but substantially incompressibleis further explained in the said application as residing in the fact that it is preferable to maintain a high transverse stiffness in the layers 4 without impairing their ability to shearin response to movement of the stiff surfaces 3, 3'; thereby to avoid a detrimental double-wall transverse resonance that would otherwisebe produced in the useful audible spectrum (say from a few hundred cycles to up to 2000 cycles, more or less) by panels of conventional construction and dimensions. The relatively stiff character of the layers 5, 3, 3', of the type of material above discussed, serves to assist in the provision of such high transverse stiffness for the medium intermediate the outer surfaces of the panel, without impairing the effective shearing qualities of the composite shearable layers 3-4-5-3'.

In the said co-pending application, suitable dimensions for attaining this end are given in an illustration of 18 gauge steel plates (i.e., of thickness of the order of tenths of an inch) and shearable layers of substantially incompressible gum rubber of about 35 Durometer, secured with an epoxy adhesive. As shown in the drawing and as described in the said co-pending application, the thickness of the shearable layers 4 is less than that of the relatively stiff layers, say, about 0.018 (i.e., of the order of hundredths of an inch).

In accordance with such a construction, the criteria before set forth of producing a composite-medium intermediate the outer surfaces of the panel that has an effective shear modulus ,u. and density p the square root of the ratio of which (Va/ determines the velocity C of shear acoustic waves in the said panel, is such that the velocity C is no greater than substantially seventenths of the velocity C of acoustic waves in the medium, such as air, surrounding the panel. High acoustic transmission loss is thus obtained, as distinguished from other sandwich panels not having such structural relationships of the elastic properties of its components. Substantially constant transverse-wave velocities have been achieved as discussed in the said co-pending application, well below the velocity of sound in air over a broad band of acoustic frequencies and void of transverse or doublewall resonance effects within such band; substantially constant velocities of the order of 450 ft. per sec. being described in the said co-pending application over the band from about 200 cycles per sec. up to about 1700 cycles per sec. Acoustic insertion loss of from about 25 to over 40 decibels over a frequency band of from a few hundred cycles per sec. up to over six thousand cycles per sec. has been experimentally obtained.

The type of sandwich construction above discussed, moreover, has an advantage over block-like cores and other intermediate media discussed in the said co-pending application and article in that all of the layers comprising the sandwich panel are substantially planar laminar layers, providing obvious advantages in fabrication.

It is to be understood, moreover, that acoustical and other advantages are attainable with panels of this construction that do not quite meet the strict acoustical requirements above set forth, although it is preferred that such acoustical criteria are met in order to attain the highest acoustic transmisison loss.

Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. An acoustic panel for providing high acoustic transmission loss over a wide band of acoustic frequencies, comprising a sandwich of at least three layers of stiff light-transmitting sheet material two of which form the outer surfaces of the panel, each layer being separated from but connected to the adjacent layer by an intermediate layer of shearable but substantially incompressible light-transmitting elastic material, the layers being substantially coextensive, the effective longitudinal stiffness of the layers of elastic material being substantially less than the longitudinal stiffness of the layers of stiff material, the effective shear modulus ,u and density p of the connected layers intermediate the said outer surfaces being related to the velocity C of shear waves in the said panel substantially by the equation C =\/,u /p; where C is no greater than substantially seven-tenths the velocity C of acoustic waves in the medium surrounding said panel, and the thickness'of said layers of stiff material and said layers of elastic material being of the order of tenths of an inch and hundredths of an inch, respectively.

2. The panel of claim 1, wherein said layers of stiff material are glass. 3. The panel of claim 1, wherein said layers of elastic material are polyvinyl plastic material.

References Cited by the Examiner UNITED STATES PATENTS 1,961,745 6/1934 Eckhardt 181--33.1 1,973,124 9/1934 Swan et al. 18133.1 2,008,655 7/1935 Clarvoe 181-33.1 2,068,782 1/ 1937 Watkins et al 161204 X 2,142,279 1/1939 Menger 161204 X 2,819,032 1/1958 Detrie et al. 181-33.1 3,007,997 11/1961 Panariti 181--33.1 3,025,198 3/1962 Dunn 181-331 3,078,969 2/ 1963 Campbell et al 181-33.1 3,079,277 2/1963 Painter 18133.1 3,087,569 4/1963 Kurtze 18133.1 3,110,369 11/1963 Ruzicka 18133.1

FOREIGN PATENTS 499,277 11/ 1950 Belgium.

LEO SMILOW, Primary Examiner.

RICHARD D. NEVIUS, Examiner. 

1. AN ACOUSTIC PANEL FOR PROVIDING HIGH ACOUSTIC TRANSMISSION LOSS OVER A WIDE BAND OF ACOUSTIC FREQUENCIES, COMPRISING A SANDWICH OF AT LEAST THREE LAYERS OF STIFF LIGHT-TRANSMITTING SHEET MATERIAL TWO OF WHICH FORM THE OUTER SURFACES OF THE PANEL, EACH LAYER BEING SEPARATED FROM BUT CONNECTED TO THE ADJACENT LAYER BY AN INTERMEDIATE LAYER OF SHEARABLE BUT SUBSTANTIALLY INCOMPRESSIBLE LIGHT-TRANSMITTING ELASTIC MATERIAL, THE LAYERS BEING SUBSTANTIALLY COEXTENSIVE, THE EFFECTIVE LONGITUDINAL STIFFNESS OF THE LAYERS OF ELASTIC MATERIAL BEING SUBSTANTIALLY LESS THAN THE LONGITUDINAL STIFFNESS OF THE LAYERS OF STIFF MATERIAL, THE EFFECTIVE SHEAR MODULUS U AND DENSTIY P OF THE CONNECTED LAYERS INTERMEDIATE THE SAID OUTER SURFACES BEING RELATED TO THE VELOCITY CS OF SHEAR WAVES IN THE SAID PANEL SUBSTANTIALLY BY THE EQUATION CS=$U/P, WHERE CS IS NO GREATER THAN SUBSTANTIALLY SEVEN-TENTHS THE VELOCITY C0 OF ACOUSTIC WAVES IN THE MEDIUM SURROUNDING SAID PANEL, AND THE THICKNESS OF SAID LAYERS OF STIFF MATERIAL AND SAID LAYERS OF ELASTIC MATERIAL BEING OF THE ORDER OF TENTHS OF AN INCH AND HUNDREDTHS OF AN INCH, RESPECTIVELY. 